1,1-Dichlorotetrafluoroethane is of interest as an intermediate to 1,1,1,2-tetrafluoroethane (i.e., CF.sub.3 CH.sub.2 F or HFC-134a) which can be obtained via catalytic hydrogenolysis of its carbon-chlorine bonds using a supported metal hydrogenation catalyst (see e.g., C. Gervasutti et al., J. Fluorine Chem., 1981/82, 19, pgs. 1-20). HFC-134a is an environmentally acceptable potential replacement for chlorofluorocarbon (i.e., CFC) refrigerants, blowing agents, aerosol propellants and sterilants that are being viewed with concern in connection with the destruction of stratospheric ozone. It is highly desired that the 1,1-dichlorotetrafluoroethane employed in the hydrogenolysis route to HFC-134a has as low a content of 1,2-dichlorotetrafluoroethane as practicable since the presence of CFC-114 during hydrogenolysis can lead to formation of 1,1,2,2-tetrafluoroethane (i.e., CHF.sub.2 CHF.sub.2 or HFC-134; see e.g., J. L. Bitner et al., U.S. Dep. Comm. Off. Tech. Serv. Rep. 136732, (1958), p. 25). HFC-134 mixed in HFC-134a may be objectionable for some applications depending on concentration and, since the two isomers boil only 7.degree. C. apart, separation of the isomers in high purity is difficult.
Commercial processes for producing C.sub.2 Cl.sub.2 F.sub.4 using either chlorofluorination of C.sub.2 Cl.sub.4 or fluorination of C.sub.2 Cl.sub.6 typically yield CFC-114 as the major isomer with CFC-114a as a minor component. Also, the precursor of CFC-114a, 1,1, 1-trichlorotrifluoroethane (i.e., CCl.sub.3 CF.sub.3 or CFC-113a) is typically produced as a minor component when its isomer, 1,1,2-trichlorotrifluoroethane (i.e, CClF.sub.2 CCl.sub.2 F or CFC-113) is manufactured using similar processes. For example, one well-known and widely-used route to the trichlorotrifluoroethanes and dichlorotetrafluoroethanes involves reaction of hydrogen fluoride (i.e., HF) with tetrachloroethylene (i.e., C.sub.2 Cl.sub.4) plus chlorine, or with its chlorine addition product, hexachloroethane (i.e., C.sub.2 Cl.sub.6), in the liquid phase in the presence of an antimony pentahalide as catalyst. The C.sub.2 Cl.sub.3 F.sub.3 and C.sub.2 Cl.sub.2 F.sub.4 products consist predominantly of the more symmetrical isomers, that is, CCl.sub. F.sub.2 CCl.sub.2 F and CCl.sub. F.sub.2 CCl.sub. F.sub.2, respectively (the symmetrical term referring to the distribution of the fluorine substituents in the molecule).
Since the boiling points of the two trichlorotrifluoroethanes and of the two dichlorotetrafluoroethanes differ only slightly from one another, separation by conventional distillation on a commercial scale is economically impractical. The lower-boiling dichlorotetrafluoroethanes (boiling range of about 3.degree.-4.degree. C.), however, are readily separable from the trichlorotrifluoroethanes (boiling range of about 46.degree.-48.degree. C.).
U.S. Pat. No. 5,055,624 discloses a process for the selective preparation of CFC-114a by fluorination of pure CFC-113a or mixtures of it with CFC-113 with anhydrous HF. The reaction is done in the liquid phase at 70.degree. to 170.degree. C., under pressure in the presence of an antimony compound of the formula SbF.sub.x Cl.sub.5-x, where x is a number from 1 to 5. In Comparative Example 8, CFC-113 was reacted with HF under a preferred set of conditions at 151.degree. C. to afford a product which contained 99.6 mole percent CFC-113 and 0.4 mole percent CFC-114. Example 4 discloses the reaction of CFC-113a with HF under similar conditions. A 99.7% yield of CFC-114a at 61.3% CFC-113a conversion was obtained.
The preparation of the trichlorotrifluoroethanes and the dichlorotetrafluoroethanes by vapor-phase reaction of HF with (A) C.sub.2 Cl.sub.4 +Cl.sub.2 or (B) CCl.sub. F.sub.2 CCl.sub.2 F over a suitable catalyst at elevated temperatures has also been well-documented in the art. As disclosed in the art, the vapor-phase processes to the C.sub.2 Cl.sub.3 F.sub.3 and C.sub.2 Cl.sub.2 F.sub.4 compounds, whatever the catalyst employed, produce a mixture of the isomers.
European Patent Application Publication No. 317,981-A2 discloses a process for producing CCl.sub.2 FCF.sub.3 which comprises isomerizing CCl.sub.2 FCCl.sub.2 F.sub.2 to form CCl.sub.3 CF.sub.3, followed by fluorination with hydrogen fluoride. In the examples, the purest CCl.sub.2 FCF.sub.3 obtained has a molar ratio of CCl.sub.2 FCF.sub.3 to CCl.sub.2 F.sub.2 CCl.sub.2 F.sub.2 of about 53:1. Also, in the examples the highest purity CCl.sub.3 CF.sub.3 feed contains about 14% CCl.sub.2 FCCl.sub.2 F.sub.2 and 86% CCl.sub.3 CF.sub.3.
There remains a need for processes to produce CFC-114a substantially free of its isomer, particularly processes which may employ conventional liquid-phase or vapor-phase fluorination techniques.